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D5014-94 – Standard Test Method Technical Guide
This article details the ASTM D5014-94 standard for the measurement of formaldehyde in indoor air using a passive sampler methodology. The standard, reapproved in 2000, specifies the collection and analysis of formaldehyde via a colorimetric method with MBTH. It covers a measurement range from 0.01 to 17 mg/m³ (0.008 to 14 ppm v/v) and sampling durations from 15 minutes to 24 hours.
🧪 Test Method Overview and Scope
The scope of D5014-94 includes personal or area measurements of formaldehyde in indoor air. The method utilizes a passive diffusion sampler with a Knudsen disk, absorbing formaldehyde into an MBTH solution. The analysis involves oxidation with ferric chloride-sulfamic acid to form a blue cationic dye, measured colorimetrically at 628 nm. The standard is applicable for concentrations from 0.01 to 17 mg/m³, with a lower quantification limit of 0.03 µg per mL of absorbing solution.
| 🟦 Parameter | 📏 Specification |
|---|---|
| Measurement Range | 0.01 to 17 mg/m³ (0.008 to 14 ppm v/v) |
| Lower Quantification Limit | 0.03 µg/mL of absorbing solution |
| Sampling Time | 15 min to 24 h |
⚙️ Sampling and Analysis Procedure
The passive sampler consists of a glass vial with a septum cap that retains a Knudsen disk, which regulates gas diffusion by Knudsen diffusion principles. During sampling, the vial is inverted to establish contact between the absorbing liquid and the disk. Formaldehyde passes into the 0.05% aqueous MBTH solution at a constant rate. After collection, the resulting azine is oxidized by a ferric chloride-sulfamic acid solution to form a blue cationic dye in acidic medium. The concentration is then measured by colorimetry at 628 nm.
| 🔍 Component | 🎯 Description |
|---|---|
| Sampler Vial | Glass vial with septum cap |
| Diffusion Disk | Knudsen disk for gas-permeable diffusion |
| Absorbing Solution | 0.05% aqueous MBTH solution |
📊 Key Measured Properties and Specifications
The standard provides specific quantitative limits. A formaldehyde concentration of 0.01 mg/m³ can be determined using 5 mL of absorbing solution for 24 hours and observing a minimum absorbance difference of 0.05 units from the blank with 1 cm path length cells. The method is subject to interference from water-soluble aliphatic aldehydes, which produce equivalent response on a molar basis. Most other interfering compounds are not gaseous or water soluble and do not pose issues.
⚠️ Warning: Water soluble aliphatic aldehydes provide significant positive interference, nearly equal to formaldehyde on a molar basis. Refer to Sections 6.2 and 10.1.7 of the standard for estimating potential interferences.
💡 Tip: Ensure the vial is inverted during sampling to maintain proper contact with the Knudsen disk. Use reagent water conforming to ASTM D1193 for solution preparation.
❓ Frequently Asked Questions
🔍 What is the measurement range of the D5014-94 test method? The test method covers formaldehyde concentrations from 0.01 to 17 mg/m³ (0.008 to 14 ppm v/v) in indoor air.
💡 How does the Knudsen disk function in the sampler? The Knudsen disk is a gas-permeable diffusion disk that regulates the transfer of gas into the absorbing solution using Knudsen diffusion principles, ensuring a constant sampling rate.
⚡ What are the key interferences in this method? Water-soluble aliphatic aldehydes interfere significantly, with response nearly equal to formaldehyde on a molar basis. Most other compounds that react to produce colored products are not gaseous or water soluble and do not interfere.
📌 What is the recommended sampling time? The recommended sampling time ranges from 15 minutes to 24 hours, depending on the expected concentration and required sensitivity. The lower quantification limit is achieved with longer sampling times.